Abstract
In this study, we analyze the variation of the aerosol optical depth (AOD) at 500 nm using measurements from January 1999 to August 2015 in the central Arabian Peninsula. The magnitudes of the annual, monthly, seasonal, and hourly trends of the AOD are studied and statistically tested using the least-squares method and supported by Mann-Kendall rank statistics at different significance levels. A significant increase of about 0.119 in the annual mean AOD is found for the entire period. AOD increased in all the seasons, in which the spring trend is the highest (0.289/17 years) and significant. The high AOD value found in spring may be attributed to the effect of dust events transported by northeasterly winds from the arid and semi-arid regions around the Arabian Gulf. In its monthly basis, AOD values in January, July, August, and September show no significant trends. On the contrary, March, April, June, and December show a clear increasing trend with different significance levels. April shows the highest increases in AOD (0.021/year), about 81.58% higher than the mean April value. Variations of the AOD on the hourly scale are investigated. Increasing trends with different significance levels are found and established. We found that the early mornings and very late afternoons presented the highest increasing trend. For the rest of the day, the AOD varies slightly due to the meteorological processes.